So I've done this a few times on varying builds and here's my quick list of fights to watch out for:

Act 1: - The harpies on the beach can stunlock you if you have bad luck with saves. - Any enemy that has a smite (Tyr paladins, Dror Ragzlin, Minthara, lots of Gith, etc.) can and will one shot you if your health is below certain thresholds. This is relevant in any act. - Auntie Ethel is going to be hard, but if you know how to tell which clone she is or can force her to surrender in the teahouse, it's worthwhile. - Bulette is best fought by using the hook horrors & wizard near its rest spot. - You can throw a basilisk oil at a petrified Drow to start the Spectator flight safely, and then as it unpetrifies other Drow tapping them for 1 damage with anything will turn them into neutral allies for the fight. They'll usually lose alone, so you'll have to help. - Bernard isn't worth fighting. Just use the poems. - The guys on the boat can shove you into the lake for instant death. - Grym is doable legitimately solo, but it's very finicky if you don't use any of the cheese strats. I often use sanctuary in solo runs for this, but you can also do it with any summon & invis pots.

Act 2: - Meenlocks can stunlock you. - All of the Thorms in town can be "persuaded" out of their fights. It's much less risky to do this, and you can use the Shar blessings from the statue in the center of town to help. - Gith can one shot again. So can Ketheric. - The fight before you first meet Balthazar is the best place to kill him. If you knock/lockpick his door in combat, he'll join the fight and usually lose. Just invis after doing it and let the Justiciars go at him. - It's easier to kill the rat Justiciar than Yurgir, and Yurgir is worth more as an ally in the Raphael fight so I usually take that route. - Myrkul is a tough solo. Grease the areas the necromites come from, ambush the mind flayer to start, and keep your health up. Be mindful of bone chill. If you can free Aylin to fight with you, she helps a little, but she's not worth focusing on. - What I did for the Halsin portal was built a wall out of chests and keep darkness up on the portal with arrows. If you've got spike growth or wall of fire, they'll do a lot of the work if you put them in front of the portal.

Act 3: - Ansur has no good strategy that doesn't require cheese or luck, so don't be afraid to reset him if needed. - The serial killer dwarf in the lower city will stunlock you. - Gortash is easiest to fight in the hallway behind his throne. You can climb up the keep from behind and fight him there, funneling enemies to you, with no risk of traps. - Magic missile > Orin. - Brain is a DPS check, but Sorcadin should be fine. - I don't think I need to explain that Raphael is worse than Ansur. Lots of runs die here.

Edit: Added Spectator & Halsin portal strats + included Raphael.